As traffic demands in communication systems continue to increase, supporting data rates beyond 100 Gbit/s will be required to increase the network capacity. A mere increase of the line rate is not viable due to bandwidth limitations in optical and electronic components and poor tolerance to signal-to-noise ratio. Multi-level modulation formats solve the former issue but exacerbate the second one. This makes modulation formats which are more complicated than, say 16 Quadrature Amplitude Modulation (16QAM) impractical. Multi-carrier transmission is therefore introduced as a straightforward option to multiply the channel capacity. Orthogonal Division Multiplexing (OFDM) is one example, where digitally generated orthogonal carriers are densely packed in order to save spectrum or, equivalently, maximize Spectral Efficiency (SE) measured in transmitted bit/s/Hz. Though OFDM is very attractive due to its high SE and the possibility of configuring bandwidth and modulation format in the digital domain, it presents drawbacks when utilised in high speed optical interfaces as disclosed, for example, in A. Barbieri et al: “Is Optical OFDM a Viable Alternative to Single-Carrier Transmission for Future Long-Haul Optical Systems?”, IEEE ICC 2010 proceedings.
On the other hand, the so called super-channels, i.e. the parallel transmission of non frequency overlapping optical carriers, though easier to implement than OFDM, generally lack spectral efficiency. This is because to avoid spectral overlap, or inter-carrier interference (ICI) the distance between adjacent carriers, ΔF, must be not lower than twice the baseband signal width, BW0, that, in practical systems, equals to a α·B, where B is the carrier baud rate and α is a roll-off factor, typically 0.6<α<0.7.ΔF≧2·BW0=2·α·B  Eq.1
SE can be improved by decreasing the baud rate B and, thus, BW0. However, this causes increased complexity of the modulation scheme, i.e.BW0=α·B=α·Rb·log2(M)  Eq. 2
Where Rb is the bit rate and M is the number of constellation symbols (e.g. M=4 for Quadrature Shift Keying (QPSK), M=8 for 8PSK and M=16 for 16QAM).
However, as mentioned above, bigger constellation sizes imply low tolerance to optical noise and, ultimately, lower achievable link distances between nodes.
An alternative approach to improve SE is increased filtering in the electrical or the optical domain. The modulated carriers before they are multiplexed and sent into the optical channel are filtered so that ΔF<2·BW0. However, time overlap of transmitted pulses, namely inter-symbol interference (ISI) is introduced. This needs to be recovered at the receiver which increases the complexity of the receiver.
This approach is known as “faster than Nyquist” transmissions, i.e. ΔF<BW0 as disclosed, for example, in G. Colavolpe: “Faster-than-Nyquist and beyond: how to improve spectral efficiency by accepting interference”, ECOC 2011 Mo-1-B-1. The working principle is illustrated in FIGS. 1a, 1b and 1c. 
As illustrated in FIG. 1a, for carrier frequencies in which ΔF=2α·B, there is no spectral overlap and hence no ISI. However, as illustrated in FIG. 1b for carrier frequencies in which ΔF<2α·B some spectral overlap occurs and hence ISI occurs. In FIG. 1c “faster than Nyquist” transmission is illustrated in which ΔF<BW0. Substantial optical or electrical filtering is applied to reduce the ISI. However, “faster than Nyquist” transmission includes a class of modulation and detection schemes which all rely on sophisticated digital signal processing algorithms. These are generally difficult to implement, especially at high transmission speeds.
Recently, however, new techniques have been introduced which present affordable implementation complexity with state-of-art integrated circuits (ICs). Such an implementation is known as “Frequency Packing” techniques. “Frequency Packing” techniques comprise nonoverlapping frequency carriers having a frequency separation ΔF lower that the baud rate B, i.e. ΔF<B.
Configuring such super-channels is further complicated by dynamic traffic allocation which is employed in existing optical networks. Individual Dense Wavelength Division Multiplexing (DWDM) channels of an optical network can be routed on new paths and wavelengths accordingly reassigned following new traffic planning or for protection needs. However, the Optical Signal to Noise Ratio (OSNR) and other propagation impairments impose restrictions on setting a new path, requiring in some cases the use of expensive Resizing, Reshaping, Retiming (3R) regeneration. As a result, adaptive transponders have been developed by which modulation format is swapped and/or the Forward Error Correction (FEC) code is changed in order to increase robustness with respect to OSNR. However, swapping modulation format, e.g. from 16QAM to QPSK, has two disadvantages. First, this increases the complexity of the modulation architecture and Digital Signal Processing (DSP) that would be needed to handle both formats. Second, smooth transitions of the line rate in swapping the formats (e.g. swapping from 16QAM to QPSK) is not possible and therefore the baud rate does not remain constant and the data rate is halved. In existing systems, a smooth transition has been achieved within a narrow data rate range by changing the FEC algorithm. The line rate is kept constant reducing the data rate but allocating more overhead (OH) for FEC.